1. Field of the invention.
The present invention relates to the field of airborne vehicle stability and control.
2. Prior Art.
A key technology to increase the overall capability of the airplane is aerodynamic control. The flight envelopes of current high performance aircraft have been limited at least in part by controllability problems at high angles of attack, typically represented by sudden departures in roll and yaw and, in some cases, by nose pitch-up or deep stall. Reduced controllability places undesirable limits on the maneuverability of the aircraft but, even worse, often leads to unrecoverable flight modes such as spins. It is therefore desirable to extend the angle of attack range for controllable flight by locally controlling the vortex flow field around the aircraft with nonconventional techniques.
The flow field around a modern high performance aircraft at moderate to high angles of attack is dominated by vortices. One of the problems on aircraft with slender forebodies such as proposed for the National Aerospace Plane is the presence of large uncontrollable yawing moments generated by asymmetric vortices on the forebody at high angles of attack, even at zero sideslip angles. The inability to overcome these large moments with conventional control surfaces has led to a number of studies to search for alternate methods of controlling the vehicle. One of the potential schemes is to effectively utilize the vortices that are the cause of the problem.
Research experiments on a series of generic high performance aircraft configurations have investigated several methods of controlling the forebody vortices including movable forebody strakes and blowing on the forebody surface. See for instance "Enhanced Controllability Through Vortex Manipulation on Fighter Aircraft at High Angles of Attack", Malcolm, G. N. and Skow, A. M., AIAA Paper No 86-22877-CP, August 1986. The typical effectiveness of the vertical tail and rudder to control the yawing moment falls off as the angle of attack increases because the vertical tail gradually becomes enveloped in the wake of the wing and fuselage. At the time the rudder effectiveness is decreasing, the asymmetric forces of the forebody vortices are increasing. If the vortices can be controlled, then they can be used for generating a controlled yawing moment to replace the lost yaw controllability from the rudder.
A number of studies have previously been conducted to investigate the effects of forebody strakes and forebody blowing. The use of forebody strakes has been shown to be an effective method of forcing naturally occurring asymmetric vortices at high angles of attack to be symmetric or nearly symmetric and to therefore eliminate the large forebody side forces and resulting yawing moments at zero sideslip. For reference see (1) "Asymmetric Aerodynamic Forces on Aircraft Forebodies at High Angles of Attack--Some Design Guides", Chapman, G. T., Keener, E. R., and Malcolm, G. N., AGARD CP-199, Conference on Stall/Spin Problems of Military Aircraft, Rhode Saint Genese, Belgium, November 1975, (2) "A Survey of Analytical and Experimental Techniques to Predict Aircraft Dynamic Characteristics at High Angles of Attack", Skow, A. M. and Titiriga, A., AGARD CP-235 Conference on Dynamic Stability Parameters, Athens, Greece, May 1975, (3) "Analysis of Wind Tunnel Data Pertaining to High Angle of Attack Aerodynamics", Headley, J. W., AFFDL-TR-78-94, Volume I, July 1978, and (4) "Exploratory Studies of Actuated Forebody Strakes for Yaw Control at High Angles of Attack", Murri, D. G. and Rao, D. M., AIAA Paper No. 87-2557-CP, August 1987). Use of asymmetrically-deployed forebody strakes has been investigated (See "Exploratory Studies of Actuated Forebody Strakes for Yaw Control at High Angles of Attack", Murri, D. G. and Rao, D. M., AIAA Paper No. 87-2557-CP, August 1987) for possible application to controlling the yawing moments. Investigations of forebody blowing techniques to control the forebody vortex orientation have also been conducted in both water and wind tunnel experiments where asymmetric forebody vortices were switched in orientation by blowing under the high vortex. For reference see (1) "Forebody Vortex Blowing--A Novel Concept to Enhance the Departure/Spin Recovery Characteristics of Fighter Aircraft", Skow, A.,.Moore, W. A. and Lorincz, D. J., AGARD CP-262, Conference on Aerodynamics of Controls, Naples, Italy, May 1979, (2) "Control of the Forebody Vortex Orientation by Asymmetric Air Injection Application to Enhance Departure/Spin Recovery", Moore, W. A., Skow, A. M., and Lorincz, D. J., AIAA Paper No. 80-0173, AIAA 18th Aerospace Sciences Meeting, Pasadena, Calif., January 1980, (3) "Control of Forebody Three-Dimensional Flow Separation, Peake, D. J., and Owen, F. K., AGARD-CP-262-15, May 1979, and "Control of Forebody Vortex Orientation to Alleviate Side Forces", Peake, D. J., Owen, F. K., and Johnson, D. A., AIAA-80-0183, January 1980, (4) "Development of Non-Conventional Control Methods for High Angle of Attack Flight Using Vortex Manipulation," Malcolm, G. N., Ng, T. T., Lewis, L. C., and Murri, D. G., AIAA Paper 89-2192, AIAA 7th Applied Aerodynamics Conference, Seattle, WA, July 31, August 1-2, 1989, (5) "Application of Forebody Blowing for Vortex Manipulation on the F-16," Ng, T. T., Eidetics International Report, 1989, and (6) "Aerodynamic Effects of Asymmetric Vortex Shedding From Slender Bodies", Ericsson, L. E. and Reding, J. P., AIAA Paper No. 85-1797, AIAA 12th Atmospheric Flight Mechanics Conference, Snowmass, CO, August 19-21, 1985.
As stated before, forebody strakes have been shown to be an effective method of forcing naturally occurring asymmetric vortices at medium to high angles of attack to be symmetric or nearly symmetric, thereby potentially favorably affecting aircraft stability, but not themselves directly providing any enhancement of aircraft control. Actuated forebody strakes, on the other hand, offer the potential for use in aircraft control. The difficulty with such strakes however, is that they tend to be complex, require hinge support structure in the forebody where no such structure now typically exists, and may require substantial actuating forces, particularly if fast response as may be required for aircraft control is to be obtained. Further, the structure, actuators, etc. required may preclude placement of the same forward of the radar, thereby forcing a further aft placement of the strakes, requiring larger strakes, increased structure and actuators, etc.